1. Field of the Invention
The present invention relates to a display device for displaying characters, waveforms, or the like by sequentially irradiating on a cathode ray tube (CRT) a display point corresponding to data which has previously been stored and, more particularly, to a display device drive circuit of a read-out oscilloscope.
2. Related Background Art
Hitherto, there has been used a method whereby two D/A converters are used and one display point is designated and displayed on the CRT. However, in this case, a digital memory or the like is used and the scale of the outer frame of the cursor is displayed so as to overlap the main waveform to be mainly observed, or the limit value or the like of this waveform is displayed together with the main waveform. In such a case, when the brightnesses of the main waveform and of the other display data are the same, there is a problem such that the main waveform to be inherently observed is hard to be discriminated.
There has conventionally been used another method whereby two D/A converters are used and one display point is designated and displayed on the CRT. However, this display method has such a problem that it is not always easy to move or enlarge the characters, waveforms, or the like which are constituted by a set of those display points.
In the case of displaying two sets of ordinary signal waveforms and characters or the like other than the ordinary signal waveforms (the characters or the like other than the ordinary signal waveforms include the data or the like which is obtained by digitizing characters, symbols, scales, and waveforms in this specification), their luminances are independently set, respectively. In this case, the ordinary signal waveforms are expressed as an analog signal, while the characters or the like other than the ordinary signal waveforms are expressed as a digital signal. However, their luminances are set by the analog levels, respectively.
For example, analog circuit of two systems are provided and their analog levels are preset, and these circuits are switched in accordance with the timing at which each of the ordinary signal waveforms and the characters or the like other than the ordinary signal waveforms is displayed.
However, the foregoing conventional switching method has the following problems. Namely, it is difficult to set the analog level because the output timings of display points to display characters or the like are irregular. The period of time to display characters or the like is short and the luminances are switched. Thus, a variation in luminances occurs or the display characters or the like are stained due to the spike signal or the like.